Enhanced Photoexcited Carrier Separation in Oxygen-Doped ZnIn2 S4 Nanosheets for Hydrogen Evolution

A Critical Review on Energy Conversion and Environmental Remediation of Photocatalysts with Remodeling Crystal Lattice, Surface, and Interface

Solar energy is a renewable resource that can supply our energy needs in the long term. A semiconductor photocatalysis that is capable of utilizing solar energy has appealed to considerable interests for recent decades, owing to the ability to aim at environmental problems and produce renewal energy. Much effort has been put into the synthesis of a highly efficient semiconductor photocatalyst to promote its real application potential. Hence, we reviewed the most advanced methods and strategies in terms of (i) broadening the light absorption wavelengths, (ii) design of active reaction sites, and (iii) control of the electron-hole (e^--h⁺) recombination, while these three processes could be influenced by remodeling the crystal lattice, surface, and interface. Additionally, we individually examined their current applications in energy conversion (i.e., hydrogen evolution, CO₂ reduction, nitrogen fixation, and oriented synthesis) and environmental remediation (i.e., air purification and wastewater treatment). Overall, in this review, we particularly focused on advanced photocatalytic activity with simultaneous wastewater decontamination and energy conversion and further enriched the mechanism by proposing the electron flow and substance conversion. Finally, this review offers the prospects of semiconductor photocatalysts in the following three vital (distinct) aspects: (i) the large-scale preparation of highly efficient photocatalysts, (ii) the development of sustainable photocatalysis systems, and (iii) the optimization of the photocatalytic process for practical application.

Defect-Tailoring Mediated Electron-Hole Separation in Single-Unit-Cell Bi3 O4 Br Nanosheets for Boosting Photocatalytic Hydrogen Evolution and Nitrogen Fixation

Solar photocatalysis is a potential solution to satisfying energy demand and its resulting environmental impact. However, the low electron-hole separation efficiency in semiconductors has slowed the development of this technology. The effect of defects on electron-hole separation is not always clear. A model atomically thin structure of single-unit-cell Bi₃ O₄ Br nanosheets with surface defects is proposed to boost photocatalytic efficiency by simultaneously promoting bulk- and surface-charge separation. Defect-rich single-unit-cell Bi₃ O₄ Br displays 4.9 and 30.9 times enhanced photocatalytic hydrogen evolution and nitrogen fixation activity, respectively, than bulk Bi₃ O₄ Br. After the preparation of single-unit-cell structure, the bismuth defects are controlled to tune the oxygen defects. Benefiting from the unique single-unit-cell architecture and defects, the local atomic arrangement and electronic structure are tuned so as to greatly increase the charge separation efficiency and subsequently boost photocatalytic activity. This strategy provides an accessible pathway for next-generation photocatalysts.

Efficient utilization of photogenerated electrons and holes for photocatalytic redox reactions using visible light-driven Au/ZnIn2S4 hybrid

In this study, a new photocatalytic reaction system for simultaneous selective oxidation of aromatic alcohols to corresponding aldehydes and reduction of protons to H₂ has been developed. The results reveal that compared with pure ZnIn₂S₄, the ZnIn₂S₄ photocatalysts modified with noble metal gold (Au/ZnIn₂S₄) significantly promote the photocatalytic performance. Among them, the 0.5% Au/ZnIn₂S₄ nanosheets shows the highest photocatalytic activity for selective oxidation of benzyl alcohol to benzaldehyde and hydrogen production, and the yields of H₂ and benzaldehyde are 326.68 and 352.04 μmol under visible light irradiation for 4 h, respectively. Those are about 4.4 and 3.6 times higher than those of pure ZnIn₂S₄ sample (74.0 μmol H₂ and 98.04 μmol benzaldehyde), respectively. The utilization ratio of photogenerated electrons to holes can achieve 92.8%. Additionally, the control experiments demonstrate that the photogenerated electrons and holes play significant roles during the reaction process. It is hoped that the current work can offer an avenue to utilize the photogenerated carriers more efficiently and to develop other photocatalytic reaction systems, such as nitrogen fixation and reduction of carbon dioxide.

Two-dimensional-related catalytic materials for solar-driven conversion of COx into valuable chemical feedstocks

The discovery of improved chemical processes for CO and CO2 hydrogenation to valuable hydrocarbon fuels and alcohols is of paramount importance for the chemical industry. Such technologies have the potential to reduce anthropogenic CO2 emissions by adding value to a waste stream, whilst also reducing our consumption of fossil fuels. Current thermal catalytic technologies available for CO and CO2 hydrogenation are demanding in terms of energy input. Various alternative technologies are now being developed for COx hydrogenation, with solar-driven processes over two-dimensional (2D) and 2D-related composite materials being particularly attractive due to the abundance of solar energy on Earth and also the high selectivity of defect-engineered 2D materials towards specific valuable products under very mild reaction conditions. This review showcases recent advances in the solar-driven COx reduction to hydrocarbons over 2D-based materials. Optimization of 2D catalyst performance demands interdisciplinary research that embraces catalyst electronic structure manipulation and morphology control, surface/interface engineering, reactor engineering and density functional theory modelling studies. Through improved understanding of the structure-performance relationships in 2D-related catalysts which is achievable through the application of modern in situ characterization techniques, practical photo/photothermal/photoelectrochemical technologies for CO and CO2 reduction to high-valuable products such as olefins could be realized in the not-too-distant future.

Ultrathin Visible-Light-Driven Mo Incorporating In2 O3 -ZnIn2 Se4 Z-Scheme Nanosheet Photocatalysts

Inspired by natural photosynthesis, the design of new Z-scheme photocatalytic systems is very promising for boosting the photocatalytic performance of H₂ production and CO₂ reduction; however, until now, the direct synthesis of efficient Z-scheme photocatalysts remains a grand challenge. Herein, it is demonstrated that an interesting Z-scheme photocatalyst can be constructed by coupling In₂ O₃ and ZnIn₂ Se₄ semiconductors based on theoretical calculations. Experimentally, a class of ultrathin In₂ O₃ -ZnIn₂ Se₄ (denoted as In₂ O₃ -ZISe) spontaneous Z-scheme nanosheet photocatalysts for greatly enhancing photocatalytic H₂ production is made. Furthermore, Mo atoms are incorporated in the Z-scheme In₂ O₃ -ZISe nanosheet photocatalyst by forming the MoSe bond, confirmed by X-ray photoelectron spectroscopy, in which the formed MoSe₂ works as cocatalyst of the Z-scheme photocatalyst. As a consequence, such a unique structure of In₂ O₃ -ZISe-Mo makes it exhibit 21.7 and 232.6 times higher photocatalytic H₂ evolution activity than those of In₂ O₃ -ZnIn₂ Se₄ and In₂ O₃ nanosheets, respectively. Moreover, In₂ O₃ -ZISe-Mo is also very stable for photocatalytic H₂ production by showing almost no activity decay for 16 h test. Ultraviolet-visible diffuse reflectance spectra, photoluminescence spectroscopy, transient photocurrent spectra, and electrochemical impedance spectroscopy reveal that the enhanced photocatalytic performance of In₂ O₃ -ZISe-Mo is mainly attributed to its widened photoresponse range and effective carrier separation because of its special structure.

Construction of noble-metal-free TiO2 nanobelt/ZnIn2S4 nanosheet heterojunction nanocomposite for highly efficient photocatalytic hydrogen evolution

A binary nanocomposite composed of two-dimensional (2D) ultrathin ZnIn₂S₄ nanosheets and one-dimension (1D) TiO₂ nanobelts was prepared and applied as a noble-metal-free photocatalyst for hydrogen evolution under solar-light irradiation. The TiO₂ nanobelt/ZnIn₂S₄ nanosheet heterojunction nanocomposites show higher light absorption capacity, larger surface area and higher separation of charge carriers in comparison to pristine TiO₂ and ZnIn₂S₄. As a result, the hydrogen production over the TiO₂/ZnIn₂S₄ nanocomposite with 15 wt% TiO₂ can reach up to 348.21 μmol · g^-1 · h^-1, even without noble metals, which is about 26 and 2.3 times higher than the pristine TiO₂ and ZnIn₂S₄, respectively. Meanwhile, a possible photocatalytic mechanism of TiO₂/ZnIn₂S₄ heterojunction nanocomposites was proposed and corroborated by photoluminescence (PL) spectroscopy and photoelectrochemical (PEC) results. This work paves a way for developing low-cost and high-efficiency noble-metal-free photocatalytic systems for solar-to-hydrogen evolution.

Hierarchical flower-like ZnIn2S4 anchored with well-dispersed Ni12P5 nanoparticles for high-quantum-yield photocatalytic H2 evolution under visible light

Ni₁₂P₅ nanoparticles were successfully anchored on hierarchical ZnIn₂S₄ through a facile solution phase route, and the Ni₁₂P₅/ZnIn₂S₄ composites manifested superior photocatalytic H₂ evolution under visible light.

The role of nitrogen defects in graphitic carbon nitride for visible-light-driven hydrogen evolution

Extending the light absorption range of g-C₃N₄ by introducing N defects may be accompanied by some negative factors for the photocatalytic activity.

Oxygen-deficient WO3via high-temperature two-step annealing for enhanced and highly stable water splitting

In contrast with conventional one-step annealed WO₃, high-temperature two-step annealed WO₃ contains a higher concentration of oxygen deficiency, leading to more efficient charge separation and improved photocatalytic ability.

Band gap tuning to improve the photoanodic activity of ZnInxSy for photoelectrochemical water oxidation

Elemental doping and band gap tuning of ZnIn_xS_y result in enhanced photoelectrochemical water splitting activity.

Catalysis with Two-Dimensional Materials Confining Single Atoms: Concept, Design, and Applications

Two-dimensional materials and single-atom catalysts are two frontier research fields in catalysis. A new category of catalysts with the integration of both aspects has been rapidly developed in recent years, and significant advantages were established to make it an independent research field. In this Review, we will focus on the concept of two-dimensional materials confining single atoms for catalysis. The new electronic states via the integration lead to their mutual benefits in activity, that is, two-dimensional materials with unique geometric and electronic structures can modulate the catalytic performance of the confined single atoms, and in other cases the confined single atoms can in turn affect the intrinsic activity of two-dimensional materials. Three typical two-dimensional materials are mainly involved here, i.e., graphene, g-C₃N₄, and MoS₂, and the confined single atoms include both metal and nonmetal atoms. First, we systematically introduce and discuss the classic synthesis methods, advanced characterization techniques, and various catalytic applications toward two-dimensional materials confining single-atom catalysts. Finally, the opportunities and challenges in this emerging field are featured on the basis of its current development.

A bottom-up synthesis of rare-earth-hydrotalcite monolayer nanosheets toward multimode imaging and synergetic therapy

Recently, ultrathin two-dimensional (2D) nanomaterials have attracted considerable research interest in biomedical applications, owing to their intriguing quantum size and surface effects. In this work, a one-step "bottom-up" method is developed to prepare rare-earth (Gd3+ and Yb3+) co-doped layered double hydroxide (LDH) monolayer nanosheets, with a precisely controlled composition and uniform morphology. Due to the successful introduction of Gd3+ and Yb3+ into the LDH host layer, the Gd&Yb-LDH monolayer nanosheets exhibit excellent magnetic resonance (MR)/X-ray computed tomography (CT) dual-mode imaging functionality. Moreover, the Gd&Yb-LDH monolayer nanosheets achieve an ultrahigh loading of a chemotherapeutic drug (SN38) with a loading content (LC) of 925%, which is a one order of magnitude enhancement compared with previously reported delivery systems of hydrophobic drugs. Interestingly, by further combination with indocyanine green (ICG), in vivo tri-mode imaging, including CT, MR and near infrared fluorescence (NIRF) imaging, is achieved, which enables a noninvasive visualization of cancer cell distribution with deep spatial resolution and high sensitivity. In addition, in vitro and in vivo therapeutic evaluations demonstrate an extremely high tri-mode synergetic anticancer activity and superior biocompatibility of SN38&ICG/Gd&Yb-LDH. Therefore, this work demonstrates a paradigm for the synthesis of novel multifunctional 2D monolayer materials via a facile "bottom-up" route, which shows promising applications in cancer synergetic theranostics.

Atomically Thin 2D Multinary Nanosheets for Energy-Related Photo, Electrocatalysis

The severe energy crisis and environmental issues have led to an increase in research on the development of sustainable energy. Atomically thin 2D multinary nanosheets with tunable components show advantages for producing sustainable energy via photo, electrocatalysis processes. Here, recent progress of atomically thin 2D multinary nanosheets from the design, synthesis, tuning, and sustainable energy production via photo, electrocatalysis processes is summarized. The regulating strategies such as alloying, doping, vacancy engineering, pores construction, surface modification, and heterojunction are summarized, focusing on how to optimize the catalytic performance of atomically thin 2D multinary nanosheets. In addition, advancements in versatile energy-related photo, electrocatalytic applications in the areas of oxygen evolution, oxygen reduction, hydrogen evolution, CO2 reduction, and nitrogen fixation are discussed. Finally, existing challenges and future research directions in this promising field are presented.

A general strategy for the functionalization of two-dimensional metal chalcogenides

Two-dimensional (2D) metal chalcogenides (MC) such as MoS₂ have been recognized as promising materials for near future applications. However, general strategies to functionalize them are still scarce, while the nature of functionalization still remains unclear. Herein, we demonstrate a simple and universal functionalization route through complexation reaction between the amino-containing organic agents and MCs. Degrees of functionalization are tunable by adjusting the organic group types and ratios. No further defects are introduced and the functionalized 2D MCs are dispersible in corresponding typical solvents. Both experimental results and geometry optimization calculations indicate that the grafting of functional groups through the coordination effect truly exist, while the surface properties and resulting photoelectric properties of 2D MCs are greatly altered. More intriguingly, our proposed functionalization process is demonstrated to be universal and can be applied to different MCs, thus opening new avenues for the application of 2D MCs.

Generalized Synthesis of Ternary Sulfide Hollow Structures with Enhanced Photocatalytic Performance for Degradation and Hydrogen Evolution

A series of ternary sulfide hollow structures have been successfully prepared by a facile glutathione (GSH)-assisted one-step hydrothermal route, where GSH acts as the source of sulfur and bubble template. We demonstrate the feasibility and versatility of this in situ gas-bubble template strategy by the fabrication of novel hollow structures of MIn₂S₄ (M = Cd, Zn, Ca, Mg, and Mn). Interestingly, with the reaction time varying, the hierarchical CdIn₂S₄ microspheres with controlled internal structures can be regulated from yolk-shell, smaller yolk-shell (yolk-shell with shrunk yolk), hollow, to solid. Under visible-light irradiation, all of our prepared CdIn₂S₄ samples with different morphologies were photoactivated. In virtue of the appealing hierarchical hollow structure, the yolk-shell-structured CdIn₂S₄ microspheres exhibited the optimal photocatalytic activity and excellent durability for both the X₃B degradation and H₂ evolution, which can be ascribed to the synergy-promoting effect of the small crystallite size together with the unique structural advantages of the yolk-shell structure. Thus, we hypothesize that this proof-of-concept strategy paves an example of rational design of hollow structured ternary or multinary sulfides with superior photochemical performance, holding great potential for future multifunctional applications.

Metallic 1T-LixMoS2 co-catalyst enhanced photocatalytic hydrogen evolution over ZnIn2S4 floriated microspheres under visible light irradiation

Metallic 1T-Li_xMoS₂ is an effective co-catalyst for photocatalytic hydrogen evolution over ZnIn₂S₄ because of its high electrical conductivity and high densities of active sites.

Elemental doping for optimizing photocatalysis in semiconductors

Among the various strategies for achieving high solar energy utilization, elemental doping has been extensively explored owing to its advantages in regulating light absorption, band positions and charge carrier processes of photocatalysts.

Noble metal-free 0D–1D NiSx/CdS nanocomposites toward highly efficient photocatalytic contamination removal and hydrogen evolution under visible light

The heterostructures formed between 1D CdS nanorods and 0D NiS_xnanoclusters were prepared and showed high photocatalytic activity.

Unique physicochemical properties of two-dimensional light absorbers facilitating photocatalysis

The emergence of two-dimensional (2D) materials with a large lateral size and extremely small thickness has significantly changed the development of many research areas by producing a variety of unusual physicochemical properties.

Ultrathin 2D Photocatalysts: Electronic-Structure Tailoring, Hybridization, and Applications

As a sustainable technology, semiconductor photocatalysis has attracted considerable interest in the past several decades owing to the potential to relieve or resolve energy and environmental-pollution issues. By virtue of their unique structural and electronic properties, emerging ultrathin 2D materials with appropriate band structure show enormous potential to achieve efficient photocatalytic performance. Here, the state-of-the-art progress on ultrathin 2D photocatalysts is reviewed and a critical appraisal of the classification, controllable synthesis, and formation mechanism of ultrathin 2D photocatalysts is presented. Then, different strategies to tailor the electronic structure of ultrathin 2D photocatalysts are summarized, including component tuning, thickness tuning, doping, and defect engineering. Hybridization with the introduction of a foreign component and maintaining the ultrathin 2D structure is presented to further boost the photocatalytic performance, such as quantum dots/2D materials, single atoms/2D materials, molecular/2D materials, and 2D-2D stacking materials. More importantly, the advancement of versatile photocatalytic applications of ultrathin 2D photocatalysts in the fields of water oxidation, hydrogen evolution, CO₂ reduction, nitrogen fixation, organic syntheses, and removal pollutants is discussed. Finally, the future opportunities and challenges regarding ultrathin 2D photocatalysts to bring about new opportunities for future research in the field of photocatalysis are also presented.

A 2D self-assembled MoS2/ZnIn2S4 heterostructure for efficient photocatalytic hydrogen evolution

Semiconductor photocatalysis for hydrogen production is a promising route to address current energy demands. It is still a great challenge to spatially separate photogenerated electrons and holes in bulk photocatalysts because of the long carrier transport pathway from the bulk to the surface. 2D heterostructured photocatalysts with the type II band alignment can not only shorten the carrier transport pathway, but also create an electric field at the interface to suppress the carrier recombination. However, ultrathin and intimate-contact 2D heterostructured photocatalysts have rarely been achieved so far. Herein, we reported that ZnIn₂S₄ nanosheets were self-assembled on few-layer MoS₂ nanosheets to fabricate ultrathin and intimate-contact 2D heterostructured photocatalysts. This 2D heterostructure was formed thanks to the strong electrostatic adsorption between MoS₂ and ZnIn₂S₄. Under visible light irradiation, the H₂ evolution rate of 2D MoS₂/ZnIn₂S₄ heterostructured photocatalysts can reach 8898 μmol g^-1 h^-1, which is almost 16 times higher than that of the pure ZnIn₂S₄ photocatalysts. The dramatically enhanced photocatalytic performance was ascribed to the better charge separation and the accelerated surface reaction due to the heterostructure and more active sites provided by MoS₂. These results provided a new insight for the design and development of 2D heterostructured photocatalysts.

Controllable Evolution of Dual Defect Zni and VO Associate-Rich ZnO Nanodishes with (0001) Exposed Facet and Its Multiple Sensitization Effect for Ethanol Detection

Building an effective way for finding the role of surface defects in gas sensing property remains a big challenge. In the present work, we synthesized the ZnO nanodishes (NDs) and first explored the formation process of rich electron donor surface defects by means of studying mechanism for the ZnO NDs synthesis. The test results revealed that ZnO-6, added by 6 mmol Zn powder, had the best gas-sensing properties with the excellent selectivity to ethanol than the others. Specially, the ZnO-6 sensor exhibited the best response (about 49) to 100 ppm ethanol at 230 °C among four as-synthesized samples, while noncustomized ZnO was only 28. It was mainly caused by the following two reasons: the exposure of target (0001) crystal facet and rich electron donor surface defects zinc interstitial (Zn_i) and oxygen vacancy (V_O). As a guide, the formation process of surface defects was revealed by an ideal defect model. By the small-angle XRD and TEM patterns, we could conclude that ZnO NDs, changing stoichiometric ratio, increased the content of Zn_i by adding Zn powder, while excessive Zn powder promoted the growth of c axis of ZnO NDs in the self-assembly engineering. Besides, a depletion model has been provided to explain how the surface defects work on the sensors and the complex mechanism of gas sensing performance. These findings will develop the application of ZnO-based gas sensor in health and security.

Hierarchical ZnIn2 S4 /MoSe2 Nanoarchitectures for Efficient Noble-Metal-Free Photocatalytic Hydrogen Evolution under Visible Light

A highly efficient visible-light-driven photocatalyst is urgently necessary for photocatalytic hydrogen generation through water splitting. Herein, ZnIn₂ S₄ hierarchical architectures assembled as ultrathin nanosheets were synthesized by a facile one-pot polyol approach. Subsequently, the two-dimensional-network-like MoSe₂ was successfully hybridized with ZnIn₂ S₄ by taking advantage of their analogous intrinsic layered morphologies. The noble-metal-free ZnIn₂ S₄ /MoSe₂ heterostructures show enhanced photocatalytic H₂ evolution compared to pure ZnIn₂ S₄ . It is noteworthy that the optimum nanocomposite of ZnIn₂ S₄ /2 % MoSe₂ photocatalyst displays a high H₂ generation rate of 2228 μmol g^-1  h^-1 and an apparent quantum yield (AQY) of 21.39 % at 420 nm. This study presents an unprecedented ZnIn₂ S₄ /MoSe₂ metal-sulfide-metal-selenide hybrid system for H₂ evolution. Importantly, the present efficient hybridization strategy reveals the potential of hierarchical nanoarchitectures for a multitude of energy storage and solar energy conversion applications.

Dual Effect in Fluorine-Doped Hematite Nanocrystals for Efficient Water Oxidation

Herein, excellent light absorption and oxygen-evolving activity were simultaneously achieved by doping fluorine anions into hematite nanocrystals. Upon anion doping, the band structure of hematite can be effectively regulated, leading to the generation of defect levels between the band gap and remarkably increased visible light absorption. The activity for electrocatalytic oxygen evolution reaction (OER) of the hematite nanocrystals is enhanced after fluorine doping, where the doped hematite assists as an effective catalyst for photoelectrochemical water splitting. The optimization strategy proposed herein may shed light on the future design of photocatalysts for energy-related applications.

Impact of Element Doping on Photoexcited Electron Dynamics in CdS Nanocrystals

Element doping plays a key role in achieving desired properties of semiconductor nanocrystals. In the energy-state landscape the doping-induced localized impurity states (LIS) can bring on significant modification of photoelectrochemical effects. It is difficult to retrieve information regarding the doping-induced LIS. Here we report on such information gleaned on a prototypical system of CdS nanocrystals slightly doped with In³⁺, through joint observations from photoluminescence (PL) and ultrafast transient absorption (TA) spectroscopy. The nonradiative nature of the In-doping induced LIS is revealed by PL. The TA observations, with a set of control experiments, enable us to capture a picture of the photoexcited electron dynamics and unravel the photoexcited electron reservoir (PEER) effect associated with the In-doping induced band gap LIS. This work establishes a fundamental, mechanistic understanding of the significant impact of element doping on the photoexcited electron dynamics in this model system, offering useful inputs for relevant material design and applications.

Ultrathin Two-Dimensional Multinary Layered Metal Chalcogenide Nanomaterials

Ultrathin two-dimensional (2D) layered transition metal dichalcogenides (TMDs), such as MoS₂ , WS₂ , TiS₂ , TaS₂ , ReS₂ , MoSe₂ and WSe₂ , have attracted considerable attention over the past six years owing to their unique properties and great potential in a wide range of applications. Aiming to achieve tunable properties and optimal application performances, great effort is devoted to the exploration of 2D multinary layered metal chalcogenide nanomaterials, which include ternary metal chalcogenides with well-defined crystal structures, alloyed TMDs, heteroatom-doped TMDs and 2D metal chalcogenide heteronanostructures. These novel 2D multinary layered metal chalcogenide nanomaterials exhibit some unique properties compared to 2D binary TMD counterparts, thus holding great promise in various potential applications including electronics/optoelectronics, catalysis, sensors, biomedicine, and energy storage and conversion with enhanced performances. This article focuses on the state-of-art progress on the preparation, characterization and applications of ultrathin 2D multinary layered metal chalcogenide nanomaterials.

High-index faceted CuFeS2 nanosheets with enhanced behavior for boosting hydrogen evolution reaction

A rational design of highly active and robust catalysts based on earth-abundant elements for hydrogen evolution reaction (HER) is essential for future renewable energy applications. Herein, we report the synthesis of a new class of ultrathin metallic CuFeS₂ nanosheets (NSs) with abundant exposed high-index {02[combining macron]4} facets. They serve as a robust catalyst for the HER with a lower onset potential of 28.1 mV, an overpotential of only 88.7 mV (at j = 10 mA cm^-2) and remarkable long-term stability in 0.5 M H₂SO₄, which make them the best system among all the reported non-noble metal catalysts. The theoretical calculations reveal that the mechanistic origin for such a high HER activity should be attributed to the excess S^2- active sites on the exposed {02[combining macron]4} high-index facets of CuFeS₂ NSs, which have a rather favorable Gibbs free energy for atomic hydrogen adsorption. The present work highlights the importance of designing ultrathin metallic chalcopyrite nanosheets with high-index facets in order to increase the number of active sites for boosting the HER performance.